When someone requests to join your team through your club's page (rather than being sent a direct invite), it lands as a pending request in your Team Admin area, ready for you to approve or reject individually.
Approving creates their player record and makes their membership active straight away. They'll be able to log in and see the team's fixtures, squad and stats immediately.
Rejecting simply leaves them unaffiliated with the team. They aren't shown an explicit rejection message, since most declined requests are duplicates or mistakes rather than anything that needs explaining.